Fix T52240: Alembic Not Transferring Materials Per Frame

When a mesh changes its number of vertices during the animation,
Blender rebuilds the DerivedMesh, after which the materials weren't
applied any more (causing the default to the first material slot).

source/blender/alembic/intern/abc_mesh.cc

@@ -983,8 +983,6 @@ static void read_mesh_sample(ImportSettings *settings,
 	if ((settings->read_flag & (MOD_MESHSEQ_READ_UV | MOD_MESHSEQ_READ_COLOR)) != 0) {
 		read_custom_data(schema.getArbGeomParams(), config, selector);
 	}
-
-	/* TODO: face sets */
 }
 
 CDStreamConfig get_config(DerivedMesh *dm)
@@ -1123,6 +1121,16 @@ DerivedMesh *AbcMeshReader::read_derivedmesh(DerivedMesh *dm,
 		CDDM_calc_normals(new_dm);
 		CDDM_calc_edges(new_dm);
 
+		/* Here we assume that the number of materials doesn't change, i.e. that
+		 * the material slots that were created when the object was loaded from
+		 * Alembic are still valid now. */
+		size_t num_polys = new_dm->getNumPolys(new_dm);
+		if (num_polys > 0) {
+			MPoly *dmpolies = new_dm->getPolyArray(new_dm);
+			std::map<std::string, int> mat_map;
+			assign_facesets_to_mpoly(sample_sel, 0, dmpolies, num_polys, mat_map);
+		}
+
 		return new_dm;
 	}
 
@@ -1133,8 +1141,11 @@ DerivedMesh *AbcMeshReader::read_derivedmesh(DerivedMesh *dm,
 	return dm;
 }
 
-void AbcMeshReader::readFaceSetsSample(Main *bmain, Mesh *mesh, size_t poly_start,
-                                       const ISampleSelector &sample_sel)
+void AbcMeshReader::assign_facesets_to_mpoly(
+        const ISampleSelector &sample_sel,
+        size_t poly_start,
+        MPoly *mpoly, int totpoly,
+        std::map<std::string, int> & r_mat_map)
 {
 	std::vector<std::string> face_sets;
 	m_schema.getFaceSetNames(face_sets);
@@ -1143,21 +1154,21 @@ void AbcMeshReader::readFaceSetsSample(Main *bmain, Mesh *mesh, size_t poly_star
 		return;
 	}
 
-	std::map<std::string, int> mat_map;
 	int current_mat = 0;
 
 	for (int i = 0; i < face_sets.size(); ++i) {
 		const std::string &grp_name = face_sets[i];
 
-		if (mat_map.find(grp_name) == mat_map.end()) {
-			mat_map[grp_name] = 1 + current_mat++;
+		if (r_mat_map.find(grp_name) == r_mat_map.end()) {
+			r_mat_map[grp_name] = 1 + current_mat++;
 		}
 
-		const int assigned_mat = mat_map[grp_name];
+		const int assigned_mat = r_mat_map[grp_name];
 
 		const IFaceSet faceset = m_schema.getFaceSet(grp_name);
 
 		if (!faceset.valid()) {
+			std::cerr << " Face set " << grp_name << " invalid for " << m_object_name << "\n";
 			continue;
 		}
 
@@ -1169,16 +1180,25 @@ void AbcMeshReader::readFaceSetsSample(Main *bmain, Mesh *mesh, size_t poly_star
 		for (size_t l = 0; l < num_group_faces; l++) {
 			size_t pos = (*group_faces)[l] + poly_start;
 
-			if (pos >= mesh->totpoly) {
+			if (pos >= totpoly) {
 				std::cerr << "Faceset overflow on " << faceset.getName() << '\n';
 				break;
 			}
 
-			MPoly &poly = mesh->mpoly[pos];
+			MPoly &poly = mpoly[pos];
 			poly.mat_nr = assigned_mat - 1;
 		}
 	}
 
+}
+
+void AbcMeshReader::readFaceSetsSample(Main *bmain, Mesh *mesh, size_t poly_start,
+                                       const ISampleSelector &sample_sel)
+{
+	std::map<std::string, int> mat_map;
+	assign_facesets_to_mpoly(sample_sel,
+	                         poly_start, mesh->mpoly, mesh->totpoly,
+	                         mat_map);
 	utils::assign_materials(bmain, m_object, mat_map);
 }
 
@@ -1234,8 +1254,6 @@ static void read_subd_sample(ImportSettings *settings,
 	if ((settings->read_flag & (MOD_MESHSEQ_READ_UV | MOD_MESHSEQ_READ_COLOR)) != 0) {
 		read_custom_data(schema.getArbGeomParams(), config, selector);
 	}
-
-	/* TODO: face sets */
 }
 
 /* ************************************************************************** */

source/blender/alembic/intern/abc_mesh.h

@@ -112,6 +112,11 @@ public:
 private:
 	void readFaceSetsSample(Main *bmain, Mesh *mesh, size_t poly_start,
 	                        const Alembic::AbcGeom::ISampleSelector &sample_sel);
+
+	void assign_facesets_to_mpoly(const Alembic::Abc::ISampleSelector &sample_sel,
+	                              size_t poly_start,
+	                              MPoly *mpoly, int totpoly,
+	                              std::map<std::string, int> & r_mat_map);
 };
 
 /* ************************************************************************** */